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Flashcards in Human Genetics - Exam 3 Deck (36)
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1
Q

Describe natural selection.

A

Natural Selection

  • The differential survival and reproduction of individuals due to differences in phenotype.
  • Fitness:
    • Is the relative reproductive success of a genotype compared to other genotypes in the population.
    • Fitness ranges from 0 to 1.
    • To calculate fitness, take the average number of offspring produced by a genotype and divide it by the mean number of offspring produced by the most prolific genotype.
  • Selection coefficient:
    • Is the relative intensity of selection against a genotype.
    • Selection coefficient equals 1 – the fitness for a particular genotype.
  • Directional selection:
    • A type of selection in which one allele or trait is favored over another.
  • The frequency of a recessive allele at equilibrium is equal to the square root of the mutation rate divided by the selection coefficient.
  • The frequency of a dominant allele at equilibrium is equal to the mutation rate divided by the selection coefficient.
  • Example:
    • Lactose-tolerant allele spread from very low frequencies to high frequencies in less than 9000 years after dairy farming produced ample supplies of milk. The estimated selection coefficient was 0.09-0.19 for a Scandinavian population. Though this selection coefficient might seem like a very small number, over evolutionary time, the favored alleles accumulate in the population and become more and more common, potentially reaching fixation.
2
Q

Describe genetic drift.

A

Genetic Drift

  • Variation in the relative frequency of different genotypes in a small population, owing to the chance disappearance of particular genes as individuals die or do not reproduce.
  • Causes of genetic drift:
    • Founder effect: the reduced genetic diversity that results when a population is descended from a small number of colonizing ancestors (e.g., Hutterites).
    • Genetic bottleneck - is a sharp reduction in the size of a population due to environmental events (such as earthquakes, floods, fires, disease, or droughts) or human activities (such as genocide).
  • Populations diverge at random in allelic frequency and can become fixed for one allele as a result of genetic drift – especially when the population is small.
3
Q

Define overdominance.

A

Overdominance (Heterozygote Advantage

  • Heterozygotes are favored over homozygotes and have a reproductive advantage which maintains both alleles in the population.
4
Q

Define underdominance.

A

Underdominance (heterozygotes selected against) - The heterozygote has a lower fitness than both
homozygotes. This leads to an unstable equilibrium.

5
Q

Describe heterzygote advantage.

A

Heterzygote Advantage

6
Q

Describe the effects of different evolutionary forces on allelic frequencies within populations.

A

Effects of evolutionary forces on allelic frequencies within populations.

7
Q

Describe biological evolution.

A

Biological Evolution

  • Genetic change in a group of organisms (Change in gene frequency in a population).
  • Two-step process.
  • Types of evolution:
    • Anagenesis - evolution taking place in a single group (a lineage) with the passage of time.
    • Cladogenesis - splitting of one lineage into two; new species arise.
8
Q

What are the levels of genetic variation?

A

Genetic Variation - Levels

  • Molecular
  • Protein
  • DNA sequence
9
Q

Describe molecular variation.

A

Genetic Variation - Molecular Variation

  • Molecular data are genetic.
  • Molecular methods can be used with all organisms.
  • Molecular methods can be applied to a huge amount of genetic variation.
  • All organisms can be compared with the use of some molecular data.
  • Molecular data are quantifiable.
  • Molecular data often provide information about the process of evolution.
  • The database of molecular information is large and growing.
10
Q

Describe protein variation.

A

Genetic Variation - Protein Variation

  • Protein variation: analyze proteins in a population to identify genotype.
  • Measures of genetic variation:
    • Proportion of polymorphic loci.
    • Expected heterozygosity.
  • Explanation for protein variation:
    • Neutral-mutation hypothesis: individuals with different molecular variants have equal fitness at realistic population size.
  • Balance hypothesis: genetic variation in natural populations is maintained by selection that favors variation.
11
Q

Describe DNA sequence variation.

A

Genetic Variation - DNA Sequence Variation

  • Restriction-site variation.
  • Microsatellite variation.
  • Variation detected by DNA sequencing.
12
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A
13
Q

Describe reproductive isolating mechanisms.

A

Reproductive Isolating Mechanisms

  • Prezygotic reproductive isolating mechanisms:
    • Gametic
    • Mechanical
    • Temporal
    • Behavioral
    • Ecological
  • Postzygotic reproductive isolating mechanisms:
    • Hybrid inviability
    • Hybrid sterility
    • Hybrid breakdown
14
Q

Describe the modes of speciation.

A

Modes of Speciation

  • Speciation: process by which new species arise.
  • Allopatric speciation:
    • When a geographic barrier splits a population into two or more groups and prevents gene flow between the isolated groups.
  • Sympatric speciation:
    • Arises in the absence of any geographic barrier to gene flow; reproductive isolation mechanisms evolve within a single interbreeding population.
  • Speciation through polyploidy.
15
Q

Describe genetic differentiation associated with speciation.

A

Genetic Differentiation Associated with Speciation

  • How much genetic differentiation is required for reproductive isolation to take place?
16
Q

Describe phylogeny.

A

Phylogeny

  • The evolutionary relationships among a group of organisms are termed a phylogeny.
  • Phylogenetic tree.
  • Phylogenetic trees are often constructed from DNA sequence data.
    • Two approaches:
      • Distance approach
      • Parsimony approach
17
Q

Describe the theories on the role of the environment in human evolution.

A

Theories on the Role of the Environment in Human Evolution

  • Adaptation to Change:
    • Assume that certain adaptations, such as upright walking or tool-making, were associated with drier habitat and the spread of grasslands, an idea often known as the savanna hypothesis.
    • According to this long-held view, many important human adaptations arose in the African savanna or were influenced by the environmental pressure of an expanding dry grassland.
    • If key human adaptations evolved in response to selection pressure by a specific environment, we would expect those adaptations to be especially suited to that habitat. Hominin fossils would be found in those environments and not present in diverse types of habitat.
  • Variability Selection Hypothesis:
    • The key events in human evolution were shaped not by any single type of habitat (e.g., grassland) or environmental trend (e.g., drying) but rather by environmental instability.
    • This hypothesis calls attention to the variability observed in all environmental records and to the fact that the genus Homo was not limited to a single type of environment.
    • Over the course of human evolution, human ancestors increased their ability to cope with changing habitats rather than specializing on a single type of environment.
    • How did hominins evolve the ability to respond to shifting surroundings and new environmental conditions?
18
Q

Describe the possible outcomes of population evolution in environmental dynamics.

A

Population Evolution in Environmental Dynamics

19
Q

How can molecular changes reveal patterns of evolution?

A

Patterns of Evolution - Revealed by Molecular Changes

  • Rates of molecular evolution:
    • Rates of nucleotide substitution.
    • Nonsynonymous and synonymous rates of substitution.
    • Substitution rates for different parts of a gene.
  • The molecular clock:
    • The rate at which a protein evolves is roughly constant over time.
    • Therefore, the amount of molecular change that a protein has undergone can be used as a clock.
  • Evolution through changes in gene regulation:
    • Genome evolution:
      • Exon shuffling.
      • Gene duplication.
        • Multigene family concept.
20
Q

Describe quantitative genetics.

A

Quantitative Genetics

  • Deals with phenotypes that vary continuously (e.g. characters such as height or mass) - as opposed to discretely identifiable phenotypes and gene-products (such as bristle number in flies, or the presence of a particular biochemical).
  • Used to identify a quantitative trait that determines oil content in corn.
21
Q

Describe discontinuous (qualitative) traits.

A

Discontinuous (Qualitative) Traits

  • Traits possess only a few phenotypes (e.g., red or white petals).
  • All of the traits Mendel studied were discontinuous.
22
Q

Describe continuous (quantitative) traits.

A

Continuous (Quantitative) Traits

  • Characteristics vary along a scale of measurement with many overlapping phenotypes.
  • For a quantitative characteristic, each genotype may produce a range of possible phenotypes. In this hypothetical example, the phenotypes produced by genotypes AA, Aa, and aa overlap.
  • 2 types:
    • Meristic characteristics.
    • Threshold characteristics.
  • Exhibit complex relationship between genotype and phenotype.
  • Are likely polygenic.
  • May have environmental influences.
  • Phenotypic ranges may overlap.
  • Cannot use standard methods to analyze.
23
Q

What is a GWAS?

A

Genome-wide association study is an examination of many common genetic variants in different individuals to see if any variant is associated (co-segregates) with a trait.

24
Q

Describe polygenic inheritance.

A

Polygenic Inheritance

  • Refers to quantitative characteristics controlled by cumulative effects of many genes.
  • Often the genes are large in quantity but small in effect.
  • Each character still follows Mendel’s rules.
  • May be influenced by environmental factors.
  • Examples of human polygenic inheritance are height, skin color, and weight.
25
Q

Describe the 2 types of quantitative characteristics.

A

Quantitative Characteristics - Types

  • Meristic characteristics:
    • Determined by multiple genetic and environmental factors, and can be measured in whole numbers.
    • Animal litter size.
  • Threshold characteristics:
    • Display only two possible phenotypes - the trait is either present or absent.
    • Quantitative because the underlying susceptibility to the characteristic varies continuously.
    • When the susceptibility exceeds a threshold value, the characteristic is expressed.
26
Q

How are quantitative characteristics analyzed?

A

Quantitative Characteristics - Analysis

  • Statistical methods are required.
  • Distribution:
    • Frequency distribution.
    • Normal distribution: a symmetrical (bell-shaped) curve.
  • Samples and populations:
    • Population: group of individuals of interest.
    • Sample: small collection of individuals from the population.
  • The Mean: the average of a set of values.
  • The Variation and Standard Deviation:
    • Variance (sd2) - the variability within a group of measurements.
    • Standard deviation: the square root of the variance.
  • The proportions of a normal distribution occupied by plus or minus one, two, and three standard deviations from the mean.
  • Correlation: when two characteristics are correlated, a change in one characteristic is likely to be associated with a change in the other.
  • Correlation coefficient (r): a statistical measure of the strength of the association.
  • Correlation does not demonstrate a cause-and- effect relation. It simply means that a change in a variable is associated with a proportional change in the other variable.
  • Regression: predicting the value of one variable, if the value of the other is given.
  • Regression coefficient: represents the slope of the regression line, indicating how much one value changes on average per increase in the value of another variable.
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30
Q

Describe heritability.

A

Heritability

  • Heritability: The proportion of the total phenotypic variation that is due to genetic difference.
  • Phenotypic Variance: Vp
    • Components of phenotypic variance Vp=VG+VE+VGE
      • Genetic variance: VG
      • Environmental variance: VE
      • Genetic x environmental Interaction VGE
    • Components of genetic variance: VG=VA+VD+VI
      • Additive genetic variance: VA
      • Dominance genetic variance: VD
      • Genic interaction variance: VI
  • Summary: Vp=VA+VD+VI+VE+VGE
31
Q

Describe the types of heritability.

A

Heritability - Types

  • Broad-Sense Heritability (H2 = VG/VP; all genetic modifiers).
    • The ratio of total genetic variance to total phenotypic variance.
  • Narrow-Sense Heritability (h2 = VA/VP; additive effects only).
    • The ratio of additive genetic variance to the total phenotypic variance.
  • Calculating Heritability:
    • Heritability by elimination of variance components:
      • (VP-VE=VG)
    • Heritability by parent-offspring regression:
      • (h2=b or h2=2b) regression against the mean of both parents [b] or 1 parent [2b].
    • Heritability and degrees of relatedness:
      • H2 = 2(rMZ-rPZ) correlation coefficient of mono vs dizygotic twins.
32
Q

Describe genetic-environmental interaction variance.

A

Genetic-Environmental Interaction Variance

33
Q

What are the important points about heredity?

A

Heredity - Important Points

  • The heritability estimate is specific to the population and environment you are analyzing.
  • The estimate is for a population, not for an individual parameter - and therefore is specific to that population.
  • Heritability does not indicate the degree to which a trait is genetic, it measures the proportion of the phenotypic variance that is the result of genetic factors for a population in a given environment.
34
Q

What are the limitations of heritability?

A

Heritability - Limitations

  • Heritability does not indicate the degree to which a characteristic is genetically determined.
  • An individual does not have heritability.
  • There is no universal heritability for a characteristic.
  • Even when heritability is high, environmental factors still influence a characteristic.
  • Heritability indicates nothing about the nature of population differences in a characteristic.
35
Q

Describe a quantitative trait locus (QTL).

A

Quantitative Trait Locus (QTL)

  • A section of DNA that correlates with variation in a quantitative trait.
  • Is typically linked to genes that control that phenotype.
  • Are mapped by identifying which molecular markers (such as SNPs or microsatellites) correlate with an observed trait.
    • This is often an early step in identifying and sequencing the actual genes that cause the trait variation (e.g., oil content in corn or muscle mass in pigs).
36
Q

Describe how genetically variable traits change in response to selection.

A

Genetically Variable Traits Change in Response to Selection

  • Natural selection arises through the differential reproduction of individuals with different genotypes.
  • Artificial selection: selection by promoting the reproduction of organisms with traits perceived as desirable.
  • Both are a consequence of successful reproduction that increases the frequency of certain alleles.
  • Predicting the Response to Selection:
    • = The extent to which a characteristic, subject to selection, changes in one generation.
  • Factors influencing response to selection:
    • S = Selection differential = [Mean phenotype of population] - [Mean phenotype of the parents selected for breeding].
  • Calculation of Response to selection = R:
    • R=h2 x S
    • h2 = narrow sense heritability (variation in a phenotypic trait in a population that is due to genetic variation between individuals in that population).